WHAT IS LOW-TEMPERATURE SUPERCONDUCTIVITY? In a normal metal, the electrons act
relatively independently of both each other and the lattice of ions.
Under certain conditions, however, the delocalised cloud can cause
indirect bonding between electrons, thereby giving current enough
coherence to
resist
any collisions or electrostatic interactions that individual
electrons may experience. In this way, the only influence on the
electrons is electron potential, a gradient of which is
provided by an electromotive force. The result of this is a
conductor without any resistance at all--a superconductor.
The temperature of the material is absolutely critical for electron
bonding. The ion lattice must have such low energy that most
of
the valence electrons remain with their associated atoms, and the
vibration of the lattice is only very slight. As a result, it is
impossible to produce this state in temperatures above about
25K.
The boiling point of liquid helium is around 4K, so this is ideal for use with
low-temperature superconductors.
Under these conditions, when an electron is liberated from an atom's
orbital, the energy of the lattice is low enough that the
moving electron exerts a significant attractive force on the
surrounding ion lattice, leaving a region of low electron potential in
its wake.
This region then attracts other electrons
causing them to follow the path of the first. As a result, an indirect
attraction between the two electrons is formed, pairing them over a
fixed distance--about a thousand times the spacing between adjacent
ions in the lattice. This attraction is only
possible if both electrons have opposed spins, which reduces the
electrostatic repulsion between them. In fact, the two electrons in an
orbital always have opposed spins and very similar energies, and the valence
orbital is destabilised under these extremely low
temperatures. This
means that most often the pairs will consist of electrons originating
form the same orbital within a very short time of each other. This
interaction between electrons is called Cooper Pairing, named
for Leon
Cooper who received the 1972 Nobel
Prize for Physics along with John Bardeen and John Schreiffer.
(Figure 6: Effect of Electrons on
Metallic Ion Lattice)
It is preferable for the electrons to remain in this state, separated
by a distance much larger than would be the case if their energy levels
dropped and they re-entered the orbital they had just left. The result
is that virtually all of the delocalised cloud consists of
paired electrons with energy only slightly higher than they would have
in the ground state.
Not only constantly interacting with each other, but also being in the
same quantum state gives the whole cloud of electrons a high degree of
coherence despite every electron not being paired to every possible
other. This means that even if one pair of electrons independently
receive enough energy to break the Cooper pair, they will not
dissociate with each other unless they also reach the next quantum
energy level, bringing a far greater degree of stability to the
superconductive state than might otherwise be expected.
However, the paired state is still relatively fragile, because the
amount of energy required to raise the quantum state is fairly trivial.
As a result, there are several considerations which ensure that a
superconductor remains in this state.
• If the temperature exceeds a critical threshold, the
electrons throughout the whole material will lose coherence due to
increased kinetic energy. This will not only physically break the
bonding within the Cooper pairs but also provide the ion lattice with
enough energy to resist attraction to passing electrons, preventing
broken pairs from reforming.
• The magnetic field applied must be below the critical field
strength, or electrons caught in it may gain kinetic energy as a result
of the motor effect. Because of this, superconductors are seldom used with alternating currents.
• Excessive charge density can lead to disruption of the lattice, causing it to be unable to mediate
the interactions between pairs of electrons. This is the result of so
many electrons being present that any difference in electron potential
created is evened out more quickly than Cooper pairs can form. In
addition, a very high current will generate a magnetic field strong
enough to destroy the superconductivity.
As soon as the pairs are separated, the material reverts to being a conventional conductor once again.
